Hardness of Different Gold Alloys

18 Carat Wedding Rings Are Harder Wearing Than 9 Carat

Chard - for the highest quality jewellery

Surprised?
Most people are surprised to hear this, because for some reason, they have always believed, or been told, that 9 carat gold is harder, or harder wearing than 18 carat gold. We sometimes have trouble convincing them, sometime they will not believe us until they see it in writing. That is why we have put it in writing, and included it in our leaflets and sales literature for some years now.
Why then does everybody tell you that 9 carat is harder?
What we have wondered for many years, is how and why people get to believe that 9 carat is harder. I believe there are two reasons why this myth has become established:-

• Ignorant Sales Staff
  Firstly, sales staff in jewellery stores do not always know their facts, and because 9 carat gold is more commonly stocked in Britain than 18 carat, they try to sell what they have in stock, therefore it is tempting to sell the benefits of 9 carat gold. I believe that staff in many multiple jewellers have long done this, probably out of ignorance; hopefully things are changing.
• The Obvious Assumption is Wrong
  Secondly, it is fairly well known that pure gold is quite soft, too soft, in fact to be used successfully in jewellery. This is quite accurate, although in some cultures, consumers prefer pure gold jewellery, but it is rather soft, so needs to be made quite solid for greater strength, and this adds to the price. Pure gold jewellery also would need to be worn with more care than alloyed gold jewellery. It is also fairly well known that if gold is alloyed with other metals, it becomes harder. Where many people make a mistake is to assume that the more "other" metals are added, the stronger and harder the resultant alloy. This is an easy and understandable mistake to make, and I believe that both consumers and many working within the jewellery trade have commonly made the same mistake over a long period of time, until the mistake has become lazily accepted as the truth.
  

Durability
This brings us to the point that hardness and durability are not one and the same thing. To give a simple example, a glass ball is harder than a rubber ball. Try throwing each onto a hard surface. The glass ball will break, but the rubber ball will bounce at remain intact, because the rubber ball is more durable than the glass one. The glass ball breaks because it is brittle. In the same way, metal alloy can also be brittle, and 9 hard carat gold alloys tend to be slightly brittle, whereas 18 carat gold alloys tend to be more resilient.

Tarnish Resistance
18 carat alloys are almost completely resistant to chemical attack in normal use, whereas 9 carat alloys are much less resistant. Nine carat alloys for example will go dull or even black merely from exposure to chemicals in the atmosphere, they will also discolour in contact with perspiration, some fabrics, bleach and other household chemicals.

Metallurgy & Alloys
To understand more fully, a little metallurgy is necessary. Each and every pure metal will have a particular hardness which will not vary much. If it is worked by rolling, stretching, bending, hammering, or other mechanical process, it will tend to become harder but more brittle. It can usually be annealed or softened by heating it. Two or more metals mixed together form an alloy. Alloys differ form pure metals, in that they often combine some of the mechanical properties possessed by their constituent metals, but often also in other less predictable ways. Many alloys for example can be hardened or softened by appropriate heat treatment. Heat treatment include heating to a variety of high or low temperatures for long or short periods of time, followed by cooling at different rates. Each alloy will have different hardness figures depending on its state. Common states to be considered include:- as cast, after casting and annealing, after age hardening (heat treatment), after cold working, after cold working followed by stress relieving, and others.

Hardness Table for Carat Gold Alloys
Although, as we have explained, there are many different alloy "recipes", to give you an indication of the hardness figures for 9 and 18 carat golds, the following table gives a range of typical "Vickers" hardness values for fairly common "recipes":-

Alloy	Hardness as Cast	Maximum Annealed Hardness
9	70 to 105	160 to 170
14	125 to 165	150 to 180
18	85 to 125	170 to 230
22	70	60 to 90
23.75	40	70
Silver	65
Platinum SC	65
Platinum HC	135

Key to Table
Silver = Typical sterling silver (925/1000)
SC = Soft casting alloy
HC = Hard casting alloy

What About 14 Carat?
Some observant folk may notice that 14 carat golds are quite hard as cast, and this is fairly accurate. In fact this can make 14 carat slightly difficult to work with in some manufacturing processes, especially for jewellers working mainly with 9 carat or 18 carat alloys. However, you will also notice that the 18 carat alloys tend to be hardenable to a higher figure than 14 carat, and providing that both standards had been correctly worked and conditioned, then the 18 carat alloys would almost always be both harder and harder wearing.

The Best
To finish with a very simple personal opinion. If there had to be only one single gold alloy purity standard, then I believe that 18 has such excellent all round properties that it would deserve to be the winner.

You may wish to visit some of our other pages:-
Allergies to Gold Jewellery
Gold Alloys by Weight & Volume
Moh's Hardness Scale
What is White Gold?
Density of Gold & Other Metals

Imitation
Perhaps we should be flattered, but the content of this page, which is our own original copyright material has been copied by at least one other jeweller in their website. We have sent requests to them to desist. The jewellers in question are:-

• Integrity Jewellers, Suite 213, 9766 Chapman Avenue, Garden Grove, California 92840, USA